Method of processing sulfate black liquor to yield methyl mercaptan and dimethyl sulfide



United States Patent 3,333,009 METHOD OF PROCESSING SULFATE BLACK LIQUOR T0 YIELD METHYL MERCAPTAN AND DIMETHYL SULFIDE James D. Wethern, Wilmington, N.C., assignor to Riegel Paper Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 22, 1965, Ser. No. 441,846

8 Claims. (Cl. 260-609) The present invention is directed to the recovery processing of waste liquors from pulp making operations, and is concerned more specifically with the treatment of spent sulfate liquors to yield useful and economic quantities of methyl mercaptan and dimethyl sulfide.

Considerable effort has been devoted in the recent past to the recovery treatment of waste liquors to yield useful byproducts, particularly methyl mercaptan and dimethyl sulfide. A variety of operable procedures has been developed for treating sulfate (kraft) and other types of waste liquors to yield quantities of methyl mercaptan and dimethyl sulfide, most of which prior precodures involve reacting the spent liquor at an elevated temperature in the presence of sulfur or various sulfur-bearing compounds. The present invention seeks to provide improved processing techniques for reacting sulfate black liquor with elemental or combined sulfur in a convenient and economic manner to achieve yields of methyl mercaptan and dimethyl sulfide which are higher and in more advantageous proportion than is realized acording to conventional processes. Thus, in addition to improved overall byproduct yield, the process of the invention is capable of giving an improved yield of relatively more valuable methyl mercaptan in proportion to dimethyl sulfide, while at the same time affording a desirable measure of control over the relative proportions of the principal byproducts to be recovered.

In accordance with more specific aspects of the invention, spent kraft black liquor is initially dried under conditions which inhibit or preclude any reaction to yield the soughtfor byproducts of methyl mercaptan and dimethyl sulfide. The drying stage is continued until the waste liquor is in a completely dry, solid, particulate form. Most advantageously, the black liquor is dried to the desired, particulate form by relatively conventional spray drying techniques,

so carried out as to produce a relatively free-flowing powder. Suitable dry powder may contain a little residual moisture; typically this would be in amounts of less than 2 percent, but could be as high as around percent.

Either before or after drying, the waste liquor has added thereto predetermined amounts of elemental or combined sulfur, sufficient to introduce from about 1 percent to about 10 percent of free sulfur, determined by weight of the free sulfur to the weight of the liquor solids. Subject to process economics, it may be appropriate in some cases to use even higher quantities of sulfur. The optimum amount of free sulfur to be added to achieve given results will vary from mill to mill and from time to time, because of variations in mill processing as well as variations in the composition of the processed pulp wood, but optimum conditions can be readily ascertained with respect to a given set of operating conditions at the mill. It has been established that the yield of the principal byproducts, methyl mercaptan and dimethyl sulfide, in accordance with the process of the invention, increases sharply with increasing amounts of added free sulfur, up to about 3 percent. Thereafter, with increasing additions of sulfur up to about 10 percent, further but lesser increases in byproduct yields are experienced with increaseingly higher amounts of added sulfur favoring the production of methyl mercaptan relative to dimethyl sulfide. Thus, as will be apparent, control over the addition of sul- 3,333,009 Patented July 25, 1967 fur affords significant process control, not only in relation to overall yields of the principal byproducts, but also in relation to the relative proportions of the yielded byproducts.

'In the procedure of the invention, sulfur may be added either in elemental form, as an inorganic sulfide, or in other combined forms in which the sulfur can react with the methoxyl groups of the lignin components contained in the liquor solids. There appears to be substantial latitude in this area of the process, although present indications are that the addition of elemental sulfur may be slightly more effective in terms of byproduct yield than the addition of equivalent amounts of sulfur in combined form.

In the process of the invention, the dry, particulate liquor solids, with homogeneously mixed sulfur or sulfur compounds, is heated in a closed reaction chamber in contact with a flowing stream of heated carrier gases to effect pyrolytic decomposition of the liquor solids. The reaction temperature, which does not appear to be particularly critical, is such that the finely divided solids are caused to melt and foam, yielding methyl mercaptan and dimethyl sulfide. The evolved byproducts are continuously carried away by a flowing stream of the carrier gas, maintained at the reaction temperature.

Carrier gases successfully employed in the process of the invention are nitrogen and superheated steam, although it is probable that other gases may be usefully employed for this purpose. The gases are preheated to reaction temperature prior to introduction into the reaction chamber so as not to draw heat from the reaction. The rate of flow of the carrier gas may be relatively low, but sufiicient to convey the evolved byproduct gases from the reaction chamber as they are formed.

In a typical process according to the invention for processing kraft black liquor to yield methyl mercaptan and dimethyl sulfide and using nitrogen or super heated steam as a carrier gas, the pyrolytic reaction has been advantageously carried out at a temperature of about 300 C. At that temperature, the desired pyrolysis of the liquor solids is effected in less than ten minutes. Some kraft liquor powders will melt and foam at temperatures significantly lower than 300 C. (e.g., 50 C. to C.), so that it may be possible to carry out the process at temperatures substantially below 300 0., although reaction time will, of course, be extended. Likewise, reaction at higher temperatures appears practical, and would be completed in relatively less time. Optimum time and temperature conditions may vary from one mill to another and from one waste liquor composition to another within the same mill. The time-temperature conditions also will be dependent on other aspects of the process, such as the precedures for disposition of the residue, etc.

In a typical experimental system for practicing the process of the invention, the carrier gas supply is directed through a glass coil superheater heated to about 300 C. in a wax bath. The heated carrier gas is then discharged directly into a glass reaction tube wound with heating wire to maintain it at about 300 C. The outlet side of the reaction tube discharged through submerged diffusion outlets (fritted glass) into a plurality of collection tubes connected in series. Each of the collection tubes was'about half full of reagent grade carbon tetrachloride, and the last (fourth) tube of the series was chilled in a Dewar flask filled with acetone and ice.

Eight samples were pyrolyzed for fifteen minutes at 300 C. in the above apparatus. Samples Nos. 14 contained 0 percent, 1 percent, 3 percent, and 10 precent elemental sulfur by weight of liquor solids, added to the liquor before drying. Samples Nos. 5-7 contained 1 percent, 3 percent, and 10 percent elemental sulfur added to the particulate material after drying. Sample No. 8 contained 3 percent elemental sulfur added to the liquor before drying. The liquor used in the samples was flash dried by a discharge from a spray nozzle into a stream of air. The air had an inlet temperature of around 200 C. and an outlet temperature of around 105 C. The residual moisture of the dried, particulate solids was around 1.6 percent. The drying of the liquor in this manner occurs so quickly that there is not time for significant evolvement of the desired reaction products to occur at this stage.

Samples Nos. 1-7 were pyrolytically reacted in the presence of nitrogen as a carrier gas, while Sample No. 8 was reacted in the presence of superheated steam as the carrier gas. The heated samples melted and foamed during the first two to three minutes, then formed a thermosetting resin residue which charred during the terminal portion of the heat. Volatile reaction products were evolved as a yellowish fog, which was substantially completely absorbed by the carbon tetrachloride of the collection tubes. Quantitative analyses for methyl mercap tan and dimethyl sulfide were made using a Beckman Model DU ultra-violet spectrophotometer. For methyl mercaptan analysis, the procedure used was as set forth by Broekhuysens article Microdosage Colorimetrique Des Thiols en Milieu Alcoolique par la N-Ethylmaleimide, Analytica Chimica Acta, XIX (1958), pp. 542-547. For dimethyl sulfide, the procedure used was as set forth by Hastingsarticle Spectrophotometric Determination of Alphatic Sulfides, Analytical Chemistry, XXV (1953), pp. 420-422. According to these determinations, the average yield of methyl mercaptan and dimethyl sulfide from the various samples was as follows:

applicability to the recovery treatment of pulping liquors derived from sulfite and other processes. Accordingly, reference should be made to the following appended claims in determining the full scope of the invention.

I claim:

1. The method of treating sulfate black liquor to yield methyl mercaptan and dimethyl sulfide, which comprises (a) removing moisture from the liquor to convert the liquor to solid, dry, particulate form,

(b) heating the particulate solids, in the presence of added sulfur in amounts up to about 10 percent of the dry solids by weight, to effect pyrolytic decomposition of the mixture, and

(c) recovering methyl mercaptan and dimethyl sulfide from gaseous decomposition products of the heated mixture.

2. The method of claim 1, further characterized by (a) pyrolytic decomposition of the mixture being carried out in the presence of a flow of heated carrier gas, and

(b) said carrier gas and gaseous decomposition products being separated and the decomposition products collected.

3. The method of claim 1, further characterized by (a) said sulfate black liquor being spray dried to solid,

particulate form in the absence of melting, and

(b) the particulate solids being heated in contact with a flowing heated carrier gas to effect decomposition.

4. The method of claim 1, further characterized by (a) said added sulfur being introduced prior to drying.

5. The method of claim 1, further characterized by (a) said added sulfur being introduced subsequent to drying and prior to pyrolytic decomposition.

The process of the invention is particularly advantageous with respect to its improved yields of methyl mercaptan. This reaction product is most desirable, not only because it commands a significantly higher price than dimethyl sulfide at current markets, but it is capable of subsequent conversion to dimethyl sulfide, if that product at any given time is more in demand.

In addition to providing improved overall yields, the process of the invention affords advantageous flexibility in the controllable proportioning of reaction products. Thus, by controlling the amounts of sulfur addition, optimum proportioning of methyl mercaptan and dimethyl sulfide may be realized, depending on the demand conditions to be satisfied at the time, and readily accommodating the expected variations in the black liquor starting material as may be expected to occur from mill to mill and even from batch to batch. 1

It should be understood that the specific procedures herein described are intended to be representative only, since certain variations may be made therein without departing from the clear teachings of the disclosure. By way of example, the procedures of the invention may have 6. The method of treating spent pulping liquor to yield methyl mercaptan and dimethyl sulfide, which comprises (a) converting the liquor to dry, solid, particulate form in the absence of significant pyrolytic reaction,

(b) causing the particulate solids to melt and foam in the presence of added sulfur, and

(c) removing and collecting evolved gases from the melted mixture.

7. The method of claim 6, further characterized by (a) said mixture being caused to melt and foam by heating to a temperature above 50 C., and

(b) said mixture being exposed to a flowing carrier gas during said heating.

8. The method of claim 7, further characterized by (a) said mixture being heated at about 300 C. for

about fifteen minutes.

No references cited.

CHARLES B. PARKER, Primary Examiner. DELBERT R, PHILLIPS, Assistant Examiner. 

1. THE METHOD OF TREATING SULFATE BLACK LIQUOR TO YIELD METHYL MERCAPTAN AND DIMETHYL SULFIDE, WHICH COMPRISES (A) REMOVING MOISTURE FROM THE LIQUOR TO CONVERT THE LIQUOR TO SOLID, DRY, PARTICULATE FORM, (B) HEATING THE PARTICULATE SOLIDS, IN THE PRESENCE OF ADDED SULFUR IN AMOUNTS UP TO ABOUT 10 PERCENT OF THE DRY SOLIDS BY WEIGHT, TO EFFECT PYROLYTIC DECOMPOSITION OF THE MIXTURE, AND (C) RECOVERING METHYL MERCAPTAN AND DIMETHYL SULFIDE FROM GASEOUS DECOMPOSITION PRODUCTS OF THE HEATED MIXTURE. 